Binders or binder systems for foundry cores and molds are well known. In the foundry art, cores or molds for making metal castings are normally prepared from a mixture of an aggregate material, such as sand, and a binding amount of a binder or binder system. Typically, after the aggregate material and binder have been mixed, the resultant mixture is rammed, blown or otherwise formed to the desired shape or patterns and then cured with the use of catalysts and/or heat to a solid, cured state.
While many different processes for forming molds or cores have been developed in the foundry industry, a fairly recent development called the "cold box" process has generated substantial interest. "Cold box" processes are carried out by passing gaseous catalysts through molded resin-coated sand at ambient temperatures in order to achieve curing. In such systems, the resinous material is generally dissolved in a solvent before it is coated on the aggregate material. "Cold box" binders and processes are disclosed in U.S. Pat. Nos. 3,905,934, 3,409,579 and 4,246,157.
The types of binders commonly used in the "cold box" system are disclosed in the aforementioned U.S patents. They are normally polyurethanes formed from polyhydroxy compounds and polyisocyanates. The polyhydroxy compounds in turn are commonly prepared from phenols and formaldehyde. While such systems have met with commercial acceptance, there is still need for a "cold box" system which exhibits improved bench life. The bench life, or time after the sand mixture is prepared that it can be used satisfactorily, is much shorter with "cold box" systems than with mixes used in other foundry processes. "Cold box" sand mixes typically begin to show substantial loss of properties 2 to 3 hours after mixing, and often the actual working life is much shorter than this, particularly in warm, humid environments.
In a typical "cold box" application, the sand is mixed with the polyhydroxy compound and the polyisocyanate in a mixer such as a muller. The sand is transported to the core blower where it is stored, usually in a hopper, and fed to the blow head as needed. The sand mixture is blown, using air pressure, into a core box and cured by passing a gaseous tertiary amine catalyst through the mix to form a cured polyurethane polymer, which bonds the sand into the desired shape. The polyhydroxy compound and polyisocyanate can react, however, even in the absence of catalysts. This slow, uncatalyzed reaction, while the sand is in the hopper and blow head, results in reduced flowability and lower tensile strength development when the cores are cured in the core box. This can lead to the production of weak, poorly compacted cores, which are prone to breakage during handling and storage. Poorly compacted cores can produce castings with unacceptable finish or with such defects as penetration or erosion. Sand in a core machine which can no longer be used must be cleaned from hoppers and the blow head, causing sand waste and machine down time. Further, cleanup at the end of a work day is much easier if the sand remaining in the blow head and hopper remains soft and does not adhere to the equipment.
Now it has been found, in accordance with this invention, that the addition of certain phosphorous compounds to the binders retards the uncatalyzed reaction between the polyhydroxy compound and polyisocyanate providing improved "cold box" binder systems with longer bench life.